But genes to appear to modulate gut microbiota composition on other animals, so it's not impossible that there is some genetic modification (to cattle) that would have the desired effect.
Another alternative is GMO feed that contain some of the same nutrients as seaweed. Gene transfer might be easier between seaweed and feed then seaweed and cow
If the inefficient cow/bacteria processing step is skipped, the whole production pipeline is greatly more climate-friendly.